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What is SU Carburetor? and How It Works?

SU Carburetor:

SU carburetor is an example of a constant vacuum type of carburetor. It consists of a single jet in which a tapered needle operates. The area of the throat is varied by means of a piston which slides up and down. The tapered needle is connected to the accelerator.

S.U. Carburetor

Working of SU Carburetor:

When the accelerator is operated, the piston moves up and down in the throat controlling the supply of air, and the needle moves up and down to the jet controlling to the supply of fuel.

su carburetor

When the piston moves up, throat area increases, for more air passes through it. The needle also moves up increasing the annular area in the jet to pass more fuel. 

When the piston moves down, throat area decreases, less air passes through it. The needle also moves down decreasing the annular area in the jet to pass less fuel.

The piston tapered needle is so designed that they maintain the correct air-fuel mixture at the different operating condition of the engine. By changing the tapered needle and adopting a suitable one, the same carburetor can be used on different H.P. engine to some extent.

  • The upper side of the piston is connected to the throttle passage through a slot cut in the piston.
  • The lower side the exposed to the atmospheric pressure.
Contant Variable Chock Carburetor

Thus, the piston at any instant depends upon the balance of its own weight (downward) against the vacuum force (upward). As the load of the piston is constant the vacuum also remains constant.

The jet can be moved up and down about to the tapered needle, by an adjusting screw fitted at the bottom of the screw. This is done for adjusting the mixture strength. There is to separate idling or slow speed system and on accelerating pump

Types of SU Carburetor

SU carburettors were provided in several “throat sizes” in both inch and millimetre measurement. These carburetors are identified by letter prefix which indicates the float type. “H”, “HD”, “HS”, “HIF”, “HV”, “OM” and “KIF”.

“H” types have float bowl with an arm cast into its base. It is mounted to the bottom of the carburetor with a hollow bolt. Fuel passes in carburetor body through the arm.

“HD” it has the float bowl with its arm fastening directly below, and concentric with the jet. The arm has a flange that tightened with four screws at the bottom of carburetor. A sealed rubber diaphragm is integral with the jet.

“HS” The main body is covered with rubber, fuel is supplied by an external pipe to the jet.

“HIF” It has horizontal Integral Float.

“HV”, “OM” and “KIF” types less commonly used.

Advantages and Uses

The main advantage of SU carburetor is the rapid response during accelerating. Hence it is fitted with racing cars and in most of the scooters and motorcycle.

SU Carburetors were widely used in automobile vehicles from much of the twentieth century. These are also used in aircraft engines including the early versions of the Rolls Royce.


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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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